WP7 opens and closes immediately

I am doing the pinball walkthrough and I find that as soon as I add the first PhysicsObjectBehavior (to the rect) to the flipper child control the applications closes as soon as it starts with the following exception. I am allowed to add a PhysicsObjectBehavior
to the flipper (and that will run) however I am blocked from adding physics objects to any other component of the flipper. The same thing works in WPF so I am wondering if there is something special for WP7 that isn't called out in the example.

The error is similar to this
http://physicshelper.codeplex.com/workitem/9728 but the steps are not the same; I am following the example to the letter and starting out with a user control, not changing after the fact and since it works in Silverlight and not WP7 I suspect the issue
is something different.

You need to be careful about where you put PhysicsObjectBehavior's (and other behaviors) - they cannot be "nested".

So you might have a Canvas with a PhysicsObjectBehavior, and inside that _another_ Canvas with a PhysicsObjectBehavior?

Instead they should be just "one level deep" - so a PhysicsControllerBehavior at the top level "game canvas" and then inside that one or more usercontrols containing PhysicsObjectBehavior's, or one or more Canvas elements with PhysicsObjectBehaviors.

I am very certain I am following the demo project exactly. If I create a Silverlight project there is no error when I create the user control, it is only when I try the same steps but in a WP7.1 project that the exception is seen. It appears
to be something specific to using the library within a WP project.

I'm using the left flipper control as a test; works in Silverlight, fails in WP7.

step 8 says: "Right-click the flipper Path and select Group Into/Canvas" and if I perform that step in a WP7.1 project it results in the exception above, leaving out the step doesn't appear to have any visual effect but the app can run without exception.
I noticed looking at the visual inheritence what the step results in a canvas within a canvas and I wonder if that has something to do with the error or not but at lease I found a solution and can move on.